As four optical isomers are known against isoleucine, it is difficult to produce L-isoleucine alone at a low cost by processes using chemical synthesis or a combination of chemical synthesis and enzymatic partition. Therefore, industrial production of L-isoleucine is mainly carried out by fermentation.
Various processes for producing L-isoleucine by fermentation are known. For example, it is known to add an L-isoleucine-precursor, such as DL-.alpha.-aminobutyric acid, .alpha.-ketobutyric acid, or threonine, to a fermentation medium or a microbial reaction system to convert the precursor to L-isoleucine. This method is known as a precursor addition method (Japanese Published Examined Patent Application Nos. 45347/60, 7091/63, 8709/68, 29789/71). However, the method is unsuitable for industrial production of L-isoleucine because of the need of expensive materials, the low yield and the unstability of the precursors.
On the other hand, as indirect fermentation methods, there are known a process by the use of microorganism belonging to Serratia marcescens having resistance to .alpha.-aminobutyric acid and isoleucine hydroxamate (J. Bacteriology, 110:761-763, 1972; Applied and Environmental Microbiology, 34: 647-653, 1977), a process by the use of a microorganism belonging to the genus Brevibacterium having resistance to .alpha.-amino-.beta.-hydroxyvaleric acid and o-methylthreonine (Japanese Published Unexamined Patent Application No. 93586/74), a process by the use of a microorganism belonging to the genus Corynebacterium having sensitivity to fluoropyruvic acid (Japanese Published Examined Patent Application No. 60236/89). Furthermore, there are known a process by the use of a microorganism belonging to Serratia marcescens which is improved by a combination of mutation and transduction (J. General Microbiology, 119:51-61, 1980), a process by the use of a microorganism belonging to the genus Escherichia or Brevibacterium which is increased activity of threonine deaminase or acetohydroxy acid synthase, key enzymes of synthesizing L-isoleucine, by using recombinant DNA technology (Japanese Published unexamined Patent Application Nos. 458/90, 42988/90).